The present invention relates to acid digestion for chemical analysis.
Acid digestion is a well understood technique for obtaining elements from compounds, various matrices that include compounds (soil), manufactured products, food products, or many other materials for which the presence and amount of one or more particular elements is of interest. The goal of digestion is to convert all element-containing species into a single form that is uniform and well defined (e.g., ions).
Uses include environmental testing of objects and structures or matrices, both natural and man-made. Examples include (but are not limited to) elements present in rocks, plant, soil, food, pharmaceuticals, plastics, or metals.
Digestion is appropriate for such elements that by virtue of composition or physical state, are not immediately susceptible to solution in water or organic solvents. Stated differently, samples that are fully soluble in water or simple organic solvents can usually be analyzed without acid digestion.
Digestion is either helpful or necessary for materials that require a significant chemical decomposition before individual elements can be identified in a solution that can be introduced into an analyzer. For total content (elemental) measurements, the digestion technique must, in combination with the chosen measurement method, allow a determination of the total element content. In a broad sense, the term “digestion” can refer to several types of techniques, including both combustion (“ashing”) and fusion (e.g., alkaline fusion). Acid digestion uses one or more strong mineral acids, often at elevated temperatures, pressure, or both, to attack the structure or matrix and reduce it to its elements. The acids must avoid contributing unwanted elements to the sample and thus should be relatively easy to purify. Within these parameters appropriate acids thus often include perchloric (HClO4), chloric (HClO3), hydrochloric (HCL), hydrofluoric (HF), nitric (HNO3), and sulfuric (H2SO4).
See, for example, Grepink, “Sample digestion for the determination of elemental traces and matrices of environmental concern,” Pure and Applied Chemistry, Vol. 61, No. 6, pp 1139-1146 (1989).
Ideally, acid digestion produces a colorless solution of ions or elements in the acid. At that point the acid solution can be cooled (if necessary) and then diluted to a concentration appropriate for the desired analytical technique. Helpful and well understood analytical techniques (particularly in the case of elemental analysis) include atomic absorption (AA) or emission (AE) spectroscopy, inductively coupled plasma (ICP), inductively coupled plasma with mass spectroscopy (ICP-MS), titration, and related tests.
Depending upon the matrix or item that the acid must attack, in some cases the digestion must be carried out in the strong mineral acids at an elevated temperature. If the required temperature is above the boiling point of the acid(s), the digestion reaction must take place in a closed vessel that will support the pressures generated at the elevated temperature.
The digestion vessel can be either helpful or problematic. In trace element analysis the vessel material must be selected to avoid both cross-contamination and losses by absorption. Fluorinated hydrocarbon polymers such as polytetrafluorethylene (PTFE) are stable and inert, but cannot withstand (at reasonable sizes) high pressure, and can gradually increase in surface area (and thus increase the potential for adsorption of some of the sample) after use. Glass and quartz-glass have some favorable properties, but are inappropriate for a silica containing matrix (which is typical in soils, rocks, plants and coal, for example) that may require digestion in HF.
CEM Corporation, the equitable and record owner of the present invention (has advanced the field of digestion significantly over several decades with products including (but not limited to) MARS6™, DISCOVER™, SP-D™, AND STAR™. These use the application of microwave energy in sealed or open vessels to carry out successful digestion faster, more conveniently, and at higher temperatures and pressures. CEM vessels include a number of advantageous features including chemical resistance and safety. U.S. Pat. Nos. 6,086,826; 6,136,276; 6,534,140; 6,287,526; 7,829,040; 8,795,608 (the illustrated vessel); and U.S. Pat. No. 9,237,608 are exemplary, but certainly not an exhaustive list. The capability of these instruments to effectively carry out digestions of materials difficult to digest has made them quite successful in the marketplace. In particular, CEM instruments can carry out digestion successfully at temperatures of at least about 150° C., and potentially as high as 180° C.
The high temperature digestion carried out as a microwave-assisted process, however, requires a much more sophisticated vessel that is typically formed of PTFE for microwave transparency and chemical resistance. The cost of a pressure resistant properly manufactured vessel for microwave assisted digestion tends to preclude these vessels from being used disposably. Additionally, in order to enhance the pressure resistant capabilities, these vessels tend to be used with a reinforcing sleeve that adds structural integrity to the vessel during the digestion; e.g. U.S. Pat. No. 6,086,826.
Metal vessels have their purposes, but in the digestion context the acid contact can also be corrosive and even small amounts of resulting contamination can lead to erroneous results. Quartz is better from a reactivity standpoint (i.e., inert), but tends not to be as safe as polymers at high temperatures, and neither quartz nor metal vessels can incorporate simplified pressure venting systems in the same manner as polymer vessels.
Some digestions can be carried out at room temperature in mineral acid, and some can be carried out at relatively moderate temperatures using open vessels heated by an instrument. As is understood by the skilled person, and the marketplace, however, the boiling point of the acid (or mixture of acids) limits the temperature to which the acids can be raised at atmospheric pressure, and thus limits the digestions that can be successfully accomplished. These modest-temperature heating systems often consist of a heated block with openings into which polyethylene or polypropylene digestion vessels can be inserted. Based on the modest price of these materials, such vessels can be used disposably in many cases.
Accordingly, a functional need and opportunity exists for improved digestions at temperatures higher than conventional lower-end heated systems, but which digestions do not require the superior capabilities (and higher cost) of high-pressure microwave assisted digestion systems.